Verbena officinalis (VO) leaf extract as an anti-corrosion inhibitor for carbon steel in acidic environment

In the present work, Verbena Officinalis (VO) leaf extract was used as potential corrosion inhibitor for the corrosion of carbon steel (CS) in 0.5 M H2SO4 medium. Further, the corrosion inhibiting nature of VO leaf extract towards the CS was evaluated using mass loss (ML), potentiodynamic polarization (PDP), electrical impedance spectroscopy (EIS) and surface morphological analyses using atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) techniques. Calculation of activation energy \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {{\text{E}}_{{\text{a}}}^{*} } \right)$$\end{document}Ea∗ using Arrhenius equation shows the increase in activation energy when adding the VO leaf extract in 0.5 M H2SO4 medium and the maximum activation energy (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{E}}_{{\text{a}}}^{*}$$\end{document}Ea∗ = 49.9 kJ mol−1) was observed for 1000 mg L−1 VO leaf extract in acid medium. The negative free energy values suggested the spontaneous and the stability of the adsorbed layer of VO leaf extract on the CS surface. Using EIS measurements, high percent inhibitory effectiveness of 91.1% for 1000 ppm solutions was achieved. With an increase in VO leaf extract dose, the double layer capacitance (Cdl) values fall while the values of charge transfer (Rct) increase. This showed that a protective layer of VO leaf extract on CS surface was formed. The polarization curves showed that the VO leaf extract acts as a mixed-type inhibitor. It is discovered that the adsorption of VO leaf extract molecules adhering to the CS surface followed the Langmuir isotherm. The anti-corrosion action of VO leaf extract is fully demonstrated by some surface techniques.


Materials and solutions
As stated by the American Iron & Steel Institute (AISI), the chemical composition of the metal used (1018 CS) is shown in Table 2.
Papers varying in grade from 250 to 1200 were used to abrade the samples, which were subsequently cleaned with bi-distilled water.The surface was buffed with acetone, and then dried with filter sheets.0.5 M H 2 SO 4 was employed as the corrosive medium in this study.It was prepared (high purity reagent grade (98%) of H 2 SO 4 was used) using bi-distilled water following standardization with Na 2 CO 3 .

Preparation of plant extract
The freshly collected plant "Verbena Officinalis of the examined plants were dried at room temperature out of sunlight and milled into a fine powder using an electrical mill.A total of 200 g of this powder was immersed in 800 ml of ethanol at a 1:4 (powder/solvent) ratio, and then put in the Soxhlet apparatus for 6 h, according to the established Soxhlet technique.The extracts were then filtered out and dried at 40 °C using vacuum filtration until dry.The yield of the dried extract was 7-10 g.The dried extracts were dissolved in DMF& ethanol (2 g/L) to form stock solution of (2000 ppm) and stored out of sunlight 37 .According to the flavonoid profile, luteolin-7-neohesperidoside and diosmetin-7-neohesperidoside are major constituents and Luteolin-7-glucoside, luteolin-7-galactoside and diosmetin-7-galactoside" also exist in adequate levels while apigenin-7-glucoside and chrysoeriol-7-galactoside present in small amounts 38 .Figure 1 represents the major chemical constituents exist in Verbena Officinalis extract.Permission: Dr. Ashraf Nofal, Lecturer of Fungi and Plant Pathology University of Sadat City, Environmental Studies and Research Institute, Sustainably Department, who provided us with VO plant, has obtained a permit from the Dean of the Institute to collect all the plants in the desert, and that is for the Research and Institute's students and that all the methods used were implemented according to the relevant regulations and guidelines.

Techniques used to compute inhibition efficiency (% IE) ML method
The ML methodology is a straightforward method for calculating the inhibitory effects of the extracts on CS in 0.5 M H 2 SO 4 .This approach employed square pieces of CS measuring (2.1 cm × 1.9 cm × 0.2 cm).The CS pieces were produced by using emery sheets with different grades of emery papers (600-2000) and then removing contaminants from the surface with acetone.The inhibition of CS corrosion was studied at various temperatures (25-50 °C) and different doses of the extract (300-1000 ppm) in unaerated solutions.Using the following equation, we calculated the corrosion rate (k corr ), surface coverage (θ), and percentage of IE 39 .
where ∆M is the Mass loss, A is the area of CS sample (cm 2 ) and t is the time (min).
The mass losses of CS with and without varying extract doses are represented by ∆M inh and ∆M free , respectively.The ML parameters were established based on the extracts' concentration and the duration of the process.

Electrochemical methods
PDP, EIS, and EFM are the three methods used to compute the % IE in electrochemical procedures.Three different electrodes are inserted into a glass cell for electrochemical experiments.The working electrodes (CS) were embedded in a resin with 1 cm 2 exposed area and treated in accordance with the earlier instructions in ML.When the electrodes were prepared to produce an open circuit potential (OCP) or steady state, it was immersed in the solution for 20 min.To minimize potential drop (IR drop), the Ag/AgCl(s) second reference electrode is positioned in close proximity to the working electrode.The third is the counter electrode, which is composed of inert platinum wire and is also referred to as the counter electrode.At OCP, the applied potential ranged from − 250 to + 250 mV with a scan rate of 0.1 mVs −1 .The electrochemical measurements were done by "Potentiostat-Galvanostat-ZRA analyzer Gamry Instrument (PCI4/750), Warminster, PA, USA) and the analysis of the data given from tests had achieved by E-Chem Analyst 5.5 software" 40 (Gamry Echem Analyst Version 5.5 software 7.8.5.8567 https:// www.gamry.com/ suppo rt-2/ techn ical-suppo rt/ insta llati on-and-setup/ insta llingechem-analy st)/.All measurements were conducted in unaerated solutions. (1)

Measurements of PDP
Using OCP scan rate of 0.1 mVs −1 (lower scan rate increases the duration of the experiments and the experiment was precise), the electrode's potential was adjusted for (PDP) measurements between − 1 and 1 V." By extrapolating Tafel slopes (β a & β c ) anodic & cathodic, the corrosion current (i corr ) was calculated.(θ) and (%IE) of the inhibitor were calculated using Eq. ( 3).
The currents of corrosion i corr and i o corr are the corrosion currents with and without the VO leaf extract.

"Measurements of EIS"
EIS tests were used at OCP with a frequency zone of (100 kHz-0.2Hz) and a voltage of 10 mV.The R ct data are utilized to calculate the (% IE) as shown in Eq. ( 4).
where R ct (inh) and R ct represent the resistances charge transfer in presence & absence of the inhibitors, respectively.

Measurements of EFM
The EFM procedure was carried out with alternating current (AC) at two distinct frequencies of 2 and 5 Hz, with 0.1 Hz as the base frequency."The intermodulation spectra's bigger peaks were utilized to determine electrochemical corrosion characteristics such as corrosion current density (i corr ), Tafel slope constants (β a and β c ), and EFM causality factors (CF-2 and CF-3).Corrosion current density (i corr ) is utilized to calculate % IE", as shown in previous Eq.( 3).

Surface inspection
The following methods are employed to assess and examine the CS surface's morphology:

AFM technique
The surface of CS samples was studied using the "Pico SPM 2100 AFM equipment (at Faculty of Engineer, Mansoura University, Mansoura, Egypt) before and after dipping in uninhibited and inhibited 0.5 M H 2 SO 4 solutions for 5 h.The inhibitor dosage utilized in this approach was 1000 ppm.Following the immersion period, the samples were removed from the test liquids and dried before being analyzed using atomic force microscope (AFM)".The analyzed CS samples' surface morphology and roughness were determined.

XPS technique
In addition to demonstrating the extracts' adsorption qualities, the protective layer that developed on the CS surface after it was dipped in 0.5 M H 2 SO 4 containing the highest dosage of VO leaf extract (1000 ppm).Also, showed the decomposition spectra of each element separately (C1s, O1s, Fe2p, Cl2p, N1s, and S2p and (BE, eV) and the assignment for each peak component were determined.There were distinct peaks at the binding energy levels of the C1s, O1s, Fe2p, Cl2p, N1s, and S2p.The adsorption of extract components on the CS surface was confirmed by recent XPS investigation data.

Method of ML
To learn more and look into how VO leaf extract concentrations affected the way that CS dissolved in 0.5 M H 2 SO 4 at different temperatures, ML tests were employed.Figure 2 shows the variation of k corr and % IE at different temperatures and concentrations of the studied extract (300-1000 ppm).Table 3 shows the corrosion parameters determined from the ML approach, and increasing the doses of VO and decrease the temperature, the percentage of IE and θ are increased.While k corr deceases this indicates that the extract molecules were adsorbed physisorption.Generally, Corrosion processes are slowed by inhibitor molecules via coating the surface of CS and producing a protective layer, minimized the accessible surface area attacked by destructive fluids and blocking corrosion sites 40 .The % IE was calculated using Eq. ( 2).The concentration of the extract and the time spent in this procedure were used to determine the ML parameters.

Activation thermodynamic parameters
The activation parameters explained the mechanism of interaction among extract compounds & CS surface.The activation thermodynamic parameters of CS corrosion were calculated using Arrhenius and transition-state Eqs.
E a * denotes activation energy, ∆H * denotes activation enthalpy, ∆S * denotes activation entropy, R represents gas constant, N represents Avogadro's number, and h represents Planck constant.
(3)  Using the Arrhenius equation, the activation energy could be determined by plot log (k corr ) vs (1/T), as shown in Fig. 3.The generated plots have straight lines with slopes of (− E a * /2.303R) and intercepts of (log A) while the other plots will have line straight with a slope of (− ∆H * / 2.303R) and an intercept of [log (R/Nh) + (∆S * /2.303R)] when log (k corr /T) plotted vs (1/T) as showed in Fig. 4".The slope and intercept were evaluated to estimate ∆H * & ∆S * values.www.nature.com/scientificreports/increased, this indicated that the addition of the VO formed a greater energy barrier to the corrosion process, and this is characteristic of physisorption inhibitors 41 .The negative value of (∆S * ), suggesting that the activated [inhibitor-metal] complex favors association over dissociation in the rate-determining phase, meaning that disordering diminishes as one progress from reactants to activated complex 42 .

Adsorption isotherm
The kind and nature of the corrosion inhibition mechanism can be ascertained using the adsorption isotherm.
To illustrate the adsorption process, numerous CS adsorption isotherms can be employed.It was found that the closest match, formula (7) 43 , used the Langmuir adsorption isotherm: where θ represents the surface coverage, C inh represents the extract dosage, K ads represents the equilibrium adsorption constant.
Figure 5 shows the graph of C/vs.C, which results in lines for VO leaf extract with, intercepts of 1/K ads and approximately unit slopes.The Eq. ( 8) 44 describes the link between the equilibrium constant of adsorption (K ads ) and the standard free energy of adsorption (ΔG°a ds ): where "(R represents the universal gas constant, T represents absolute temperature, and (55.5) is the water concentration in M)".
In 0.5 M H 2 SO 4 , the free energy of adsorption for adsorbed OV extract on the CS surface is equivalent to 38.1-50.1 kJ mol −1 , and the log K ads ranges from 4.9 to 6.5.The fact that ΔG°a ds is negative demonstrates the spontaneous adsorption of VO leaf extract on the CS surface 45 .Chemisorption, which involves the sharing or transfer of charges from inhibitor molecules to the metal surface to form a coordinate type of bond, is involved in negative ones larger than 40 kJ mol −146,47 , whereas physisorption, which is the electrostatic interaction between charged particles and the charged metal, is typically represented by ΔG°a ds around − 20 kJ mol1 or higher 48 .It is possible that the VO leaf extract adsorption process on CS in 0.5 M H 2 SO 4 solution is mixed kind inhibitor but predominately chemisorption because the estimated value for ΔG°a ds is greater than − 40 kJ mol −1 .Furthermore, the standard enthalpy (∆H o ads ) and the standard entropy (∆S o ads ) data may be determined using the Van't Hoff Eq. ( 9) by relation between (log K ads /T) vs (1/T) as illustrated in Fig. 6 and thermodynamic general Eq. ( 10): Table 5 displays the data, which indicates that the values of ∆G o ads are negative and around 20 kJ mol −1 .This indicates that the adsorption of VO leaf extract on the CS surface occurred both spontaneously and physically.The enthalpy values of adsorption (∆H o ads ) show that the adsorption of VO leaf extract on the CS surface followed a physisorption process and was exothermic.When VO leaf extract was adsorbing onto the CS surface, entropy and disorder were decreased, as seen by the entropy (∆S o ads ) values for the extract being negative.The potential for an unconstrained solution is seen to decline over time and stabilize at − 486 mV SCE after 100 s.This behavior can be explained by the corrosive compounds that accumulate on the surface of the CS as it deteriorates.As a result, when VO leaf extracts are present at concentration between 300 and 1000 ppm, the potential first decreases, then rises, and finally becomes quickly stable over time.The breakdown of the oxide coating and the development of a protective film on the metallic surface can both be used to explain this phenomenon.In inhibited solutions, the potential obtained moved to values more positive than those observed in uninhibited solutions, according to a careful inspection of the OCP curves..The PDP graphs of CS in the presence and absence of modified doses from VO leaf extract at 25 °C are displayed in Fig. 8.It can be shown that the cathodic & the anodic reactions are shifted to the positive and negative directions, respectively.Table 6 "list the PDP parameters (i corr , E corr , β a and β c ) and show that: the corrosion current (i corr ) decreases as the dosage of examined extract increases.Corrosion rate (k corr ) was reduced by increasing the extracts dosage.The (% IE) and θ increased with increasing the dose of examined extracts due to the creation of a protecting layer on the CS surface, demonstrating the inhibitors' validity.Anodic and cathodic reactions were suppressed as the dose of VO leaf extract rose, with a cathodic impact being more pronounced.When, the investigated VO leaf extract was added, the Tafel slopes for VO (β a , β c ) were slightly altered.The inhibitor coating therefore had the same impact on cathodic and anodic reactions.The activation barriers for anodic and cathodic reactions are measured by the Tafel slopes β a and β c , respectively.As previously stated 52 , the minimal change in E corr required to classify an inhibitor as either cathodic or anodic must be ± 85 mV.The greatest displacement in the current investigation, 27 mV, showed that the VO leaf extract was a mixed-type inhibitor, inhibiting both cathodic and anodic processes by blocking their active sites on the metal's surface.The parallel Tafel lines show that the mechanism is unaffected by the addition of VO leaf extract.Values of i corr reduced as extract amounts grew, indicating that the % IE increased with increasing inhibitor dose and showing reasonable consistency with the outcomes of other approaches.

EIS tests
The goal of the EIS studies was to provide insight into the features and kinetics of electrochemical processes happening at the CS/0.5 M H 2 SO 4 interfaces with and without VO leaf extract.Figure 9 presents the Nyquist and Bode plots of the impedance responses of this system.The resulting Nyquist plots are depressed semicircles with the center located below the x-axis.Solid electrodes are rarely precisely round, and surface roughness, active site distribution, and other homogeneities have been commonly associated with frequency dispersion 53 .A single capacitive loop makes for the majority of Nyquist graphs.According to the capacitive loop seen in both inhibited and uninhibited solutions, the charge transfer mechanism (also known as activation control) was primarily responsible for controlling corrosion 54 .The width of the capacitive loop grew with the VO leaf extract   where ω max signifies the angular frequency when the impedance imaginary component is at its highest value, "Y 0 is the magnitude of CPE, and n is a CPE exponent" is dependent on the Table 7 shows the electrochemical parameters calculated from EIS measurements as (R ct , C dl , θ, % IE).The R ct value, which measures the flow of electrons from the metal to the electrolyte, is inversely related to the rate of corrosion.The corrosion rate was   www.nature.com/scientificreports/minimized as the electron transport between the metal surface and the corrosive liquid was suppressed as the VO leaf extract dosages rose, as shown by an increase in R ct values.As VO leaf extract dosages were increased, C dl readings tended to drop.The addition of the inhibitor changes the composition and structure of the electric double layer.The organic VO leaf extract components that were adsorbed on the metal surface replaced some of the pre-adsorbed water molecules, which decreased C dl 55 and decreased the dielectric constant.The decrease in C dl 56 might have also been caused by the shrinkage of the electrolyte-containing region brought about by the inhibitor coating's growth.The thickness of the film produced increases as a result of more inhibitor molecules adhering to the surface at increasing inhibitor concentrations.As a result, to monitor the inhibitor's adsorption, the Cdl may be assessed both before and after the corrosion inhibitor is administered.

EFM measurements
Two sine waves with frequencies of (2, 5 Hz) are applied to the cell for EFM measurements in presence & absence of different extracts dosages VO leaf extract.The findings are generated immediately with this procedure; hence the Tafel constants are not required.The output current is non-linear and depends on the applied frequency.The causality factors acquired from EFM tests are highly essential because they demonstrated the validity of the EFM measurements if the data of (CF-2 & CF-3) are around theoretical values (2, 3).Harmonica peaks in the output spectra of current provide the data of k corr .The bigger peaks are used for determination of the current density (i corr ), Tafel slopes (β c & β a ), and causality factors (CF-2 & CF-3).Equation ( 3) used for determination of % IE. Figure 11 shows the EFM spectra of CS in the presence and absence of extracts from VO leaf extract in 0.5 M H 2 SO 4 .Table 8 shows the results, and we can see that: As the inhibitor concentration was raised, the current density (i corr ) dropped; the causality factors derived from testing are equivalent to theoretical values.Increasing the dosage of the studied extracts reduces the k corr rate while increasing the inhibition efficiency (% IE).When corrosion rates and inhibition efficiency are calculated using chemical methods and those results are contrasted with those obtained using electrochemical techniques, certain variations are always apparent.Even at low doses of the inhibitor, it was seen that the % IE values obtained from impedance measurements were typically greater than those obtained from weight loss and potentiodynamic polarization tests.This may be explained by the fact that when mild steel is submerged in acid for an extended period of time, cathodic hydrogen evolution increases, probably because more cathodic sites are exposed to the corrosion process 57 .This might be a potential explanation for why such rapid electrochemical methods, even at low inhibitor doses, provide large values of % IE.On the other hand, longer periods of immersion provide lower values of % IE than those from impedance measurements in other techniques including weight loss and potentiodynamic polarization tests.However, at higher levels of inhibition, there is better agreement between the results obtained using the various techniques.This is likely because the higher bulk concentration of the inhibitor causes an increase in adsorption, which allows for the establishment of equilibrium conditions in milliseconds or less.

AFM analysis
Surface topography images produced by AFM with atomic or near-atomic resolution enable the estimation of the observed species' surface roughness.The three-dimensional (3D) AFM morphologies for the CS surface in 0.5 M H 2 SO 4 solutions with and without the VO leaf extract are shown in Fig. 12 for comparison.The roughness average values are 130.27 and 995.24 nm, respectively.Average metal surface roughness in 0.5 M H 2 SO 4 is greater than for shielded CS.According to these results, molecules from VO leaf extract bond to CS surfaces and create a shielding layer that effectively guards the surface from damaging ions 58 .

XPS analysis
High-resolution XPS was used to evaluate the variations that occur on the CS surface through the corrosion operation, both in the presence and lack of the VO leaf extract.The survey XPS spectra for CS surface in the lack and presence of VO leaf extract are shown in Fig. 13, showing the XPS destruction peaks for each element independently, which are detected in the surface layer created in a solution that regulates the presence of the extracts' contents.The CS metal peaks obtained when dipped in a 0.5 M H 2 SO 4 containing the greatest dosage of VO (1000 ppm) were for C1s, O1s, Fe2p, Cl2p, N1s, and S2p.The binding energies (BE, eV) and the assignment for every peak component are shown in Table 9.At the binding energy levels of the C1s, O1s, Fe2p, Cl2p, N1s, and S2p different peaks were found in 0.5 M H 2 SO 4 and 1000 ppm of VO leaf extract".The extract components' adsorption on the CS surface was validated by the most recent XPS investigation data.

Mechanism of corrosion inhibition
The plant extracts adsorption on CS surface is demonstrated using a variety of techniques, including chemical procedures, electrochemical techniques, and surface analysis.The extracts of VO leaf extract produce a protecting layer on CS surface, closing active spots and minimizing corrosion by inhibiting the destructive media.As previously indicated, an inhibitor's action mechanism in corrosive environments begins with adsorption of the inhibitor on the metal surface.Adsorption is influenced by a number of variables, including the inhibitor's chemical makeup, the metal's composition and surface charge, the quantity and variety of adsorption sites, the nature of aggressive electrolytes, and the interaction of the inhibitor's organic molecules with the metallic surface 59 .It should be noted that CS has a positive surface charge in acidic settings [60][61][62] and that this should typically prevent the adsorption of protonated species, which are the main components of VO leaf extract in acidic conditions and include luteolin and diosmosing -7-neohesperidoside.As previously indicated, the resulting surface charge adjustment considerably changes the nature of the metal-inhibitor interactions, therefore the capacity of certain anions in solution to get particularly adsorbed on the positively charged metal surface is a crucial factor.As a result, the presence of cooperative adsorption between the cationic species and SO 4 2− ions might be used to explain the excellent inhibitive characteristics of VO leaf extract in HCl.Joint adsorption may be divided into two categories:

Figure 2 .
Figure 2. Time-ML diagrams for the liquefaction of CS in 0.5 M H 2 SO 4 in the presence and lack of different doses of VO at 25 °C, 180 min.

Figure 3 .Figure 4 .
Figure 3. Log k corr vs. 1/T for CS in attendance & absence of VO in 0.5 M H 2 SO 4 .

Figure 5 .
Figure 5. Langmuir isotherm of VO leaf extract of CS dissolution from ML test at altered temperatures.

Figure 6 .
Figure 6.Log K ads vs 1/T for the adsorption of VO leaf extract on CS in 0.5 M H 2 SO 4 at (25-50 °C).

Figure 8 .
Figure 8. PDP bends of CS dissolution in presence & absence of altered dosages of VO leaf extract at 25 °C.
dosage, increasing corrosion resistance, although the Bode curves contain two loops connected to one another.Complex plane plot analysis was carried out by fitting the experimental findings to the equivalent circuit shown in Fig.10in order to acquire reliable results.To better mimic the non-ideal capacitive behavior of the double layer, a constant phase element (CPE) is employed in place of the double layer capacitance (C dl ) in the circuit, which is composed of solution resistance (R s ) in series with the parallel combination of charge transfer resistance (R ct ).A number of physical processes, such as inhibitor adsorption, the formation of porous layers, surface inhomogeneity caused by surface roughness, etc., are quantified by the Y o component of the CPE.Double layer capacitance (C dl ) is taken into consideration using the relationship below:

Figure 10 .
Figure 10.A simple circuits used to suit the EIS results.

Figure 13 .
Figure 13.XPS plots for CS at 0.5 M H 2 SO 4 in presence of 1000 ppm from VO leaf extract.

Table 1 .
Investigations on plant extracts as metal and alloy corrosion inhibitors.

Table 2 .
CS sample chemical composition:

Table 4
displays the derived activation parameter data.The results of activation thermodynamic parameters showed the E a * of the corrosion process of CS in 0.5 M H 2 SO 4 acid increased as the examined VO dosage

Table 4 .
CS activation parameters with and without VO leaf extract in 0.5 M H 2 SO 4 .

Table 6 .
PDP parameters of CS in 0.5 M H 2 SO 4 over altered dosages of VO leaf extract at 25°C.

Table 7 .
EIS test results of CS dissolution in 0.5 M H 2 SO 4 at altered doses of VO leaf extract.

Table 8 .
Electrochemical kinetic parameters of CS dissolution EFM test in 0.5 M H 2 SO 4 solutions attendance and lack various doses of the used VO leaf extract.

Table 9 .
BE (eV) for the significant core lines observed on the CS surface.